Quartz crystals are widely used to provide reference frequencies in electronic oscillators. The frequency at which a quartz crystal resonator vibrates depends on its physical dimensions. Moreover, changes in temperature causes the quartz crystal to expand or contract due to thermal expansion and changes in the elastic modulus of quartz. The physical changes in turn change the crystal oscillation frequency. Although quartz has a very low temperature coefficient of frequency, temperature changes are still the major cause of frequency variation in crystal oscillators.
Oven controlled crystal oscillators (“OCXO”) are frequency reference devices where the quartz oscillator is placed inside a temperature controlled oven. The oven is provided to maintain the oscillator at a constant temperature in order to prevent changes in the frequency due to variations in ambient temperature. This type of oscillator achieves the highest frequency stability possible with a quartz crystal. OCXOs are typically used to control the frequency of radio transmitters, cellular base stations, military communications equipment, and devices for precision frequency measurements, for example.
For OCXOs, the oven is a thermally-insulating enclosure that contains the crystal and one or more electrical heating elements. Since other electronic components in the oscillator circuit are also vulnerable to temperature drift, usually the entire oscillator circuit is enclosed in the oven. For these devices, a temperature sensor, such as thermistor, will be provided to monitor the oven temperature and a closed-loop control circuit will be provided to control the power to the heater to maintain the oven at the precise target temperature. Since the oven operates above ambient temperature, the oscillator usually requires a warm-up period of several minutes after the power has been applied. Moreover, the frequency of the device will not have the full rated stability during this warm-up period.
Although existing OCXOs generally provide good stability (e.g., typically better than 100 parts per billion (“ppb”) over a specified temperature range), these devices also have several shortcomings. First, a typical quartz crystal is fairly large, which, turn, makes the final OCXO devices quite large. Since the manufacturing cost of the timing device is proportional to the size, the larger OCXO size is not preferred. Second, the long thermal time constant for heating and cooling leads to a very long start-up time. For example, it typically takes several minutes to stabilize the oven at the target temperature. Third, the power needed to maintain the oven temperature is fairly large. For example, a typical OCXO consumes over 1 watt to heat the oven. Finally, due to temperature gradients in the oven, the crystal temperature is not constant but may change by +/−1 K over an ambient temperature range of −40 to 85 C.
In general, microelectromechanical system (“MEMS”) resonators are small electromechanical structures that vibrate at high frequencies and are often used as an alternative to quartz crystals. Accordingly, oven controlled MEMS oscillators that may provide very small timing devices with fast start-up times could offer significant benefits in many applications. Unfortunately, MEMS processes are not sufficient for realizing high accuracy clock without extensive physical trimming.
The accuracy challenge of such oven controlled MEMS oscillator devices is illustrated in FIG. 1, for example. In general, a typical MEMS resonator intended for oven controlled MEMS oscillator applications has a parabolic temperature dependency with a nominal turnover temperature Tturn=85° C. This means that when the device is placed in an oven heated to temperature of 85° C., small oven temperature fluctuations around this turnover temperature will only a small effect on the oscillation frequency. For example, a typical parabolic temperature dependency for MEMS resonator is −40 ppb/K2. Thus, if the oven temperature varies by +/−0.5K around the turnover temperature Tturn 85° C., the corresponding oscillation frequency variation is just 10 ppb which is acceptable for most applications.
However, due to manufacturing variations of MEMS resonators, the turnover temperature may vary by +/−5K as shown in FIG. 1, which increases the oscillator frequency variation. For example, as shown, if the turnover temperature is 90° C. and the oven temperature is 85° C., oven temperature fluctuations of +/−0.5K may lead to +/−200 ppb frequency fluctuations. This variation in oscillation is too significant for high accuracy clock applications.